1. Field of the Invention
The present invention relates to a technology for performing image processing on image data by determining a character area and a picture area of the image data.
2. Description of the Related Art
In most image processing devices that identify character areas and picture areas and apply different image processing to the character or the picture areas discretely, an area that is identified as a character area and is subjected to image processing for characters is, in most cases, a character edge area. When the line width is up to about 10 dots, it is not difficult to determine that the area including the inside is a character area. When the line width is larger, it is difficult to determine that the area including the inside is a character area. In this case, image processing for characters is applied to the edge area and image processing for pictures is applied to the inside area, the differences in color and brightness between the two areas becomes visually identifiable, which degrades the picture quality.
In the devices described in Japanese Patent Application Laid-Open Publication Nos. 2000-134471 and 2003-189090, to minimize the differences between the processing applied to a character inside area and that applied to a character edge area for the purpose to make the visual degradation less remarkable, a character edge area, a character inside area, and a picture area are distinguished from each other, and proper image processing such as the ink process and the γ conversion process is applied to the identified areas respectively. For the character inside area, image processing is applied using image processing parameters having substantially intermediate characteristics between the characteristics of image processing parameters for character edges and the characteristics of image processing parameters for pictures. In Japanese Patent Application Laid-Open Publication No. 2003-189090, exactly the same parameters as the parameters for character edge are used in the inking process.
Conventionally, a K simple color inking process for reproduction only with a black color material is applied to an edge of a black character. This scheme is employed to prevent coloring at an edge section due to occurrence of a color shift between a black color material and other color materials when output from a printer. In Japanese Patent Application Laid-Open Publication Nos. 2000-134471 and 2003-189090 as described above, as shown in FIG. 18, K simple color process or inking process close to the K simple color process is applied to inside of a black character.
When an inside area of a high density black character is reproduced using the K simple color process or a process close to the K simple color process, density of the character inside area in the script is not reproduced as it is and is apt to be reproduced slightly brighter, which is disadvantageous. It is generally known that reproduction by adding CMY color materials to a K simple color is more effective than reproduction by using a K simple color for using a lower brightness range of black effectively. Reproduction using not only K but also CMY is performed when a more realistic image is desirable in printing an image, for instance, on photographic paper. What is described above is also applicable to black characters, and the inking process is closely related to an image depth.
Basically, the problem due to the differences in colors and brightness between a character edge area and a character inside area occurs in bold characters having low to medium density. When a black-character edge area is reproduced using a K simple color (a process for a character) and a black character inside area is reproduced using both K and CMY colors (a process for a picture), the result is as shown in the left side of FIG. 19, and inside of the character is reproduced with C, M, and Y color materials and the K color material is not used although all of the K and C, M, and Y color materials should be used. In this case, it becomes difficult to conform colors and brightness in the edge area to those in the character inside area, and it is almost impossible when considering the changes over time. On the other hand, the higher the density in the character inside area is, a larger amount of the K color material is used in the character inside area like in the character edge area, which makes it more difficult to follow the changes. Besides such problems relating to colors and brightness as described above, a white spot may occur in a bold character having low density due to color shift when output, as shown in the right side in FIG. 19. In a bold character having medium density, degradation becomes larger because of a portion having low density although the color is not completely white. In a case of a bold character having high density, however, degradation is smaller even when a color shift occurs. Namely, when processing for a picture is applied to a character inside area alone, troubles of color, brightness, and a white spot occur only in a bold character having low and medium density, and the troubles do not occur in a bold character having high density. On the other hand, when processing for characters is applied, there is the risk that a portion of a picture is wrongly detected as a character inside area to cause serious degradation.